1. Field of the Invention
This invention relates to the so-called wet developing method for visualizing an electrical latent image formed in electrophotography, electrostatic recording or the like, namely, an electrostatic latent image, with a liquid developer, and to a wet developing device for carrying out such a method.
In the art of image formation such as electrophotography, electrostatic printing, electrostatic recording and the like, various developing methods have been known and used to visualize or develop an electrostatic latent image on a latent image carrier such as a photosensitive medium using a photoconductive substance or an electrostatic recording material. In these known developing methods there is generally used so-called toner, namely, electroscopic particles which are more or less selectively drawn to or repulsed from the electrostatic charge of the latent image. When such toner is applied to the surface on which a latent image is carried, the developer particles adhere to the latent image area for direct reproduction or adhere to the area outside of the latent image for reversible reproduction.
When the developing is carried out with developer in a form of a dry powder it is called dry developing method and when the developing is carried out with liquid developer prepared by dispersing such toner in a carrier liquid, it is called a wet developing method. Both of these developing methods are widely known at present. Compared with the dry developing method, the latter mentioned method, that is, the wet developing method has the advantage that the apparatus required for carrying out the method is relatively simple in structure and that troubles involved in carrying out the method such as scatter of developer can be minimized. Therefore, the wet developing method is preferably used for a simple type image forming apparatus rather than the dry method.
The wet developing method may be defined as such developing method which comprises the step of making liquid developer contact the latent image carrying surface. The liquid developer is prepared by dispersing toner particles in a suitable dielectric carrier liquid having a volume resistivity greater than about 10.sup.10 .OMEGA. cm and a dielectric constant less than 3 (for example, a paraffin hydrocarbons). When brought into contact with the latent image carrying surface, the toner particles dispersed in the carrier liquid are drawn to the electrostatic latent image and adhere thereto so as to visualize the latent image.
2. Description of the Prior Art
There are a great variety of wet developing methods such as cascade method, immersion method, method using a jet of developing liquid, or method of supplying developing liquid by the use of a roller formed of a humidity-containing material such as sponge, felt or the like. In these arts of wet development, high speed has recently become a particularly important goal. To accomplish such high-speed development, developing liquid of high concentration must be supplied to the electrostatic image bearing surface at high speed and excess developing liquid must be collected efficiently. In addition, it is important to provide a visualized image of good quality from which excess developing liquid has been sufficiently removed. As a system which substantially satisfies the several needs in such a wet developing art, there is known a method in which an elastic roller or belt of liquid-retaining property having a liquid-passing surface is used as the developing liquid supply means and development of the electrostatic image on the electrostatic image bearing member is effected at the region whereat the roller or belt is urged against the image bearing member. Details of this wet developing method are disclosed in Japanese Patent Laid Open No. 40336/1977. According to this method, at the region whereat the elastic roller or belt (hereinafter referred to as the elastic roller) is urged against the electrostatic image bearing member, squeeze-out and supply of developing liquid and press-out of excess developing liquid are effected at the same time in accordance with the elastic deformation of the elastic roller itself and thus, the time required for the development is short and in this point, this method is found to be advantageous over the other known methods such as the wet developing method using an immersion bath or the wet developing method using a jet of developing liquid. Incidentally, the aforementioned Japanese Patent Laid Open No. 40336/1977 describes that, as an example, an elastic roller comprising a sponge roller covered with a flexible netting is used as developing means. Such elastic roller has the property of squeezing out liquid previously absorbed therein and of absorbing liquid thereinto by being rotatively urged against the surface of a rigid member such as photosensitive medium or insulating member so as to be elastically deformed and thereby form a desired width of nip. Therefore, where such roller is used as the developing means for electrostatic image, the usefulness thereof would be sufficiently recognized. In the course of studies carried out to apply the above-described elastic roller for the development of electrostatic image in accordance with such knowledge, the inventors have found several merits and demerits of the wet developing method using such an elastic roller. That is, according to this developing method, development, defogging and removal of excess developing liquid are accomplished simultaneously in the developing step and this has the advantage that the time required for development is very short, that no edge effect appears in the visualized image, and that copying on a long footage of copy paper or continuous copying at short intervals is possible. On the other hand, this roller developing method has the disadvantage that the visualized image obtained thereby lacks sharpness and that the visualized image is low in resolving power.